Organic electroluminescent (EL) devices are expected to see practical application in such fields as displays and lighting. Various research is being carried out on materials and device structures with the aim of achieving such properties as low-voltage driving, high brightness and longevity.
The increased functionality of organic EL devices is being achieved via a multilayer stacked structure consisting of functional layers that are functionally discrete. Such functional layers are broadly divided by function into three types: hole-injecting/transporting layers, light-emitting layers, and electron-injecting/transporting layers. Of these, the most important function carried out by hole-injecting/transporting layers is to take hole carriers that have been injected through the anode from an external power source and transport them efficiently to the light-emitting layer. In order for these hole carriers and the electron carriers injected from the cathode side to efficiently recombine within the light-emitting layer, the function of preventing electron carriers from flowing out to the hole-injecting/transporting layer from the light-emitting layer is also required.
Film-forming processes for organic functional layers are broadly divided into dry processes such as vapor deposition and wet processes such as spin coating. On comparing these processes, wet processes are able to produce thin films having a high flatness over a large surface area, and are superior for reducing costs and achieving larger surface areas. In particular, there is a strong desire to achieve a wet process approach to the formation of a hole-injecting/transporting layer that can serve as a common layer underlying the light-emitting layer.
In light of such circumstances, the inventors have developed charge-transporting materials which give thin films capable of achieving excellent characteristics when employed in the hole-injecting layer of organic EL devices, compounds with a good solubility in organic solvents for use in such materials, and charge-transporting varnishes (Patent Documents 1 to 4).
In order to be able to impart the brightness characteristics of an organic EL device, a higher uniformity is desired not only in the hole-injecting layer but also in, for example, the hole-transporting layer (Patent Document 5). In addition, there exists a desire for a material which provides a charge-transporting thin film of excellent flatness and which can also achieve an excellent coatability for the hole-transporting layer or light-emitting layer that is formed on this film by a wet process.